Process for treating a gas oil

ABSTRACT

The present invention relates to a process for hydrodesulphurising a gas oil wherein a sulphur level of less than 50 ppm is obtained, said process comprising a) contacting a gas oil with hydrogen in the presence of a hydrodesulphurisation catalyst, and b) adjusting the temperature and pressure of hydrodesulphurisation so the temperature-pressure relationship lies on or between the curves T=211.15 P 0.1363  and T=243.83 P 0.1232 , for all values of P≧0 where T=temperature in ° C., and P=pressure in bar a.

[0001] The present invention relates to a process for the hydrodesulphurisation of gas oil.

[0002] Sulphur is present throughout the boiling range of petroleum fractions in the form of different organic sulphur compounds. In the naphtha to gas oil boiling range (100 to 400° C.), these sulphur compounds can be classified as one of the following sulphur types: mercaptans, sulphides, di-sulphides, thiophenes, benzothiophenes (BT) and di-benzothiophenes (DBT). It is desirable to remove such compounds from petroleum fractions for technical and environmental reasons. This may be carried out by hydrodesulphurisation.

[0003] In a typical hydrodesulphurisation process, the petroleum fraction is reacted with hydrogen in the presence of a metal-oxide catalyst at a temperature of 280 to 410° C. (P=10-100 bars). During this reaction, the sulphur compounds are converted to hydrogen sulphide gas, which is easily removed. Generally, however, aromatic sulphur species such as thiophene, BT, and DBT are more difficult to hydrodesulphurise than aliphatic sulphur compounds (e.g. mercaptans, sulphides, di-sulphides).

[0004] The degree of substitution within each sulphur type can also have a large impact on the ease of hydrodesulphurisation. Thus, alkylated DBTs having the formula (I) below tend to be more difficult to treat than DBT itself

[0005] where R=ethyl or methyl

[0006] Particularly difficult to hydrodesulphurise are compounds having the formula (II) below:

[0007] In this specification, such compounds (I) will be referred to as hindered DBTs.

[0008] In gas oil fractions (typical bp 200 to 380° C.), the majority of the sulphur is present as BT, DBT and hindered DBT. Although the sulphur content of gas oil may be reduced to 500 ppm by conventional hydrodesulphurisation processes, attempts to reduce the sulphur content of gas oil below 500 ppm have proved relatively uneconomic. This is because at low sulphur concentrations (eg below 500 ppm) a significant proportion (eg 80-100%, usually 95-100%) of the sulphur present in gas oil is in the form of hindered DBT. As these compounds are difficult to hydrodesulphurise, they are generally removed from the feedstock by excluding the heavier fractions of gas oil (bp>330° C.) where they are found, in order to achieve very low product sulphur contents. However, exclusion of the heavier fractions of gas oil reduces the amount of product available to the diesel pool in the refinery, so is generally undesirable.

[0009] We have now found that it is possible to hydrodesulphurise hindered DBT at an optimum rate by adjusting the temperature and pressure of hydrodesulphurisation process to within predetermined limits, thereby enabling production of gas oil having a sulphur content of less than 50 ppm without the need to remove heavier fractions.

[0010] According to the present invention, there is provided a process for hydrodesulphurising a gas oil wherein a sulphur level of less than 50 ppm is obtained, said process comprising a) contacting a gas oil with hydrogen in the presence of a hydrodesulphurisation catalyst, and b) adjusting the temperature and pressure of hydrodesulphurisation so the temperature-pressure relationship lies on or between the curves T=211.15 P^(0.1363) and T=243.83 P^(0.1232), for all values of P≧0 where T=temperature in ° C., and P=pressure in bar a.

[0011] Preferably, the temperature-pressure relationship lies on or between the curves, T =215.79^(0.1342) and T=234.45 P^(0.1267), more preferably, between the curves T=215.79P^(0.1342) and T=229.78 P^(0.1285). In one embodiment of the invention, the temperature for any given pressure is within +/−2° C. of the curve T=225.11P^(0.1304). The above-mentioned six curves are plotted in FIG. 1 below, with the scope of the invention being defined by the two outer curves. It should be understood that the letters “y” and “x” in FIG. 1 are used to denote T and P, respectively.

[0012] The hydrogen employed in step a) may be pure hydrogen. However, impure hydrogen may also be used. In the case of the latter, it should be understood that P= partial pressure of hydrogen in bara.

[0013] Preferably the hydrodesulphurisation is operated at a temperature between 270 to 430° C., preferably 290 to 400° C., and most preferably 320 to 390° C., and at a pressure between 5 and 80 bara, preferably 10 and 70 bara, and more preferably between 10 and 60 bara. In one embodiment, the pressure is between 20 and 35 bara. In a preferred embodiment, the pressure is between 20 and 30 bara.

[0014] In the present invention, the concentration of sulphur compounds in the gas-oil is reduced by contacting the gas oil with hydrogen in the presence of a hydrodesulphurisation catalyst. During the hydrodesulphurisation reaction, sulphur is released from the sulphur compounds in the gas oil in the form of hydrogen sulphide. The temperature and pressure conditions employed in the present process are particularly useful for reducing the sulphur content of the gas oil to below 50 ppm.

[0015] Without wishing to be bound by any theory, the present invention is based on the finding that at a temperature range conventionally employed for hydrodesulphurisation, the hydrodesulphurisation of hindered DBTs may be kinetically, or thermodynamically controlled, depending on the specific temperature employed. This is in contrast with the hydrodesulphurisation reactions of other sulphur compounds such as BT and DBT, which are thought to proceed under kinetic control throughout that temperature range. Under kinetic control, the rate of hydrodesulphurisation increases with increasing temperature. Beyond a certain temperature however (which temperature depends on the pressure employed), thermodynamic control takes over and the rate of hydrodesulphurisation decreases with increasing temperature. Thus, whereas the rate of hydrodesulphurisation of DBT and BT increases with increasing temperature, the rate of hydrodesulphurisation of hindered DBT goes through a maximum at a particular temperature. In other words, for a given pressure, there is believed to be an optimum temperature for hydrodesulphurisation of hindered DBT to occur. When the sulphur concentration of gas oil has been reduced to approximately 500 ppm, the majority of the sulphur remaining in the gas oil is in the form of hindered DBT. Thus, by carefully adjusting the temperature and pressure conditions of the hydrodesulphurisation process to remove a greater proportion of the hindered DBT, sulphur levels can be reduced to below 50 ppm.

[0016] The process of the present invention may be used to treat any gas oil boiling in the range of 200 to 380° C. Preferably, the process may be used to treat gas oil having T₉₅>300° C., most preferably T₉₅>345° C., and especially T₉₅>360° C. Examples of gas oils include light gas oils (LGO), heavy gas oils (HGO), light cycle oils (LCO), coker gas oils (CGO), Visbroken gas oils (VBGO), and mixtures thereof. These oils may contain sulphur compounds, such as BT, DBT and hindered DBT.

[0017] The process is capable of being used to reduce the sulphur level of the gas oil to less than 50 ppm, preferably less than 40 ppm, most preferably, less than 30 ppm, and especially less than 10 ppm. Typically, no BT and DBT are present in the final product; almost all the remaining sulphur being in the form of hindered DBT.

[0018] The process of the present invention may be carried out in a reactor comprising a fixed bed hydrodesulphurisation catalyst. The gas oil and hydrogen may be introduced into the reactor separately, for example, via two separate inlets or via a single inlet as a gas oil/hydrogen mixture. Preferably, the ratio of gas oil to hydrogen employed is 5:1 to 1000:1, preferably 100:1 to 500:1, and most preferably 150:1 to 350:1 (nm³/m³, measured at 0° C.). This corresponds to a hydrogen partial pressure at reactor outlet of 0 to 200 bara, preferably, 15 to 80 bara and most preferably, 20 to 50 bara. The gas oil and hydrogen may be pre-heated prior to contacting the catalyst.

[0019] The gas oil and hydrogen may be passed over the catalyst at a liquid hourly space velocity of 0.1 to 10, preferably, 0.5 to 2h⁻¹. In the presence of the hydrodesulphurisation catalyst, the hydrogen reacts with the gas oil, to release sulphur in the form of hydrogen sulphide, and optionally, also to saturate at least some of the unsaturated components present in the gas oil. Once treated, the gas oil may be cooled, and preferably, introduced into a separator where any unreacted hydrogen gas is removed. The unreacted hydrogen is preferably recycled for re-use.

[0020] Any suitable hydrodesulphurisation catalyst may be used in the present invention. Such a catalyst may comprise an active component which is dispersed on a catalyst support. Examples of active components include molybdenum and tungsten compounds. Molybdenum sulphide is preferred. Optionally, a catalyst promoter may be used in combination with the catalyst. Examples of catalyst promoters include cobalt and nickel. The active component, and optional promoter may be supported on any suitable catalyst support, sucks silica, and/or gamma alumina. Where a gamma alumina support is employed, it may also comprise amounts of silica and/or phosphorus.

[0021] In one embodiment of the present invention, the hydrodesulphurisation process is carried out in a single fixed-bed reactor, in which the hydrodesulphurisation catalyst is located at a lower portion and/or outlet of the reactor. The gas oil and hydrogen is introduced into an upper portion of the reactor, which may be operated at a temperature of, 270 to 430° C., preferably, 290 to 400° C., and most preferably, 320 to 390° C. This temperature may or may not fit the area between the curves T=243.83 P^(0.1232) and T=211.15 P^(0.1363). The temperature at the lower portion of the reactor, however, is — maintained at a value defined by the area of the two curves. For example, at a pressure of 30 bar, the hydrodesulphurisation temperature at the lower portion of the reactor is between 335 and 370° C.

[0022] In an alternative embodiment of this invention, the hydrodesulphurisation reaction is carried out in a multi-bed reactor. Such a multi-bed reactor comprises a number of fixed bed reactors which are coupled to one another in series. Typically, a plurality of fixed bed reactors are coupled together in series. In use, the reactor effluent from one reactor is introduced (either directly or indirectly) into the next reactor in the series for treatment, with or without the means of reducing the gas and oil temperature, generally known as “quench”. This quench may be cool hydrogen containing gas, a hydrocarbon liquid such as diesel feed or product, or an external means of heat removal such as a heat exchanger. In this way, the catalyst bed temperatures of a multibed reactor may be controlled to operate within the optimal operating temperature.

[0023] Another way of using this invention is to adjust the hydrogen partial pressure at any given catalyst bed temperature to stay within the two outer curves in FIG. 1. This may be done by several means, such as improving the hydrogen content of the makeup gas, adding a sponge oil system to remove excess hydrocarbons from the recycle gas or by operating a gas purge from the recycle gas system. In this way the temperature pressure relationship defined by the region between the outer curves in FIG. 1 may be used to optimise the destruction of hindered DBT's and so facilitate the desulphurisation of gas oil.

EXAMPLE

[0024] A feedstock (density=0.8305 g/^(cm3)) having the composition defined in Table 1 below was contacted with hydrogen in the presence of a hydrodesulphurisation catalyst. The feedstock was passed over the catalyst at a liquid hourly space velocity (lhsv) of 2.29. The initial sulphur content of the feed was 91 ppm (0.417%). The initial partial pressure of hydrogen in the reactor was 25.6 bara, although this dropped to 23.1 bara during the course of reaction.

[0025] To test the effect of temperature on the rate of sulphur removal, the weighted average bed temperature (WATB) of the catalyst bed was increased from 328.3 to 354.0° C., and the concentration of sulphur in the feedstock was measured. As shown in Table 2 and FIG. 2, the concentration of sulphur in the feedstock continued to fall sharply, until the WATB temperature of the bed reached 345.7° C. At this temperature, the S content of the feedstock was 31.5 ppm. By heating the feedstock above this temperature, the sulphur content decreased slightly, but not to any significant degree. Thus, 345.7° C. was considered as the optimum temperature for reaction. This optimum falls between the lines T=243.83 P^(0.1232) and T=211 5 P^(0.1353), i.e. within the scope of the present invention. TABLE 1 Density Feed Initial Boiling Point 142.6  5% v/v 188.4 10.0 204.0 20.0 226.1 30.0 244.2 40.0 259.0 50.0 271.4 60.0 282.8 70.0 294.6 80.0 308.6 90.0 327.1 95.0 342.3 Final Boiling Point 353.3

[0026] TABLE 2 WABT ppm S 328.3 91.0 339.3 58.5 345.7 31.5 353.0 28.5 353.0 27.0 354.0 27.0 

I claim:
 1. A process for hydrodesulphurising a gas oil wherein a sulphur level of less than 50 ppm is obtained, said process comprising a) contacting a gas oil with hydrogen in the presence of a hydrodesulphurisation catalyst, and b) adjusting the temperature and pressure of hydrodesulphurisation so the temperature-pressure relationship lies on or between the curves T=211.15 P^(0.1363) and T=243.83 P^(0.1232), for all values of P>0 where T=temperature in ° C., and P=pressure in bar a.
 2. A process as claimed in claim 1, wherein the temperature-pressure relationship lies on or between the curves T=215.79P^(0.1363) and T=234.45P^(0.1267).
 3. A process as claimed in claim 1, wherein temperature-pressure relationship lies on or between the curves T=215.79 P^(0.1342) and T=229.78 P^(0.1285).
 4. A process as claimed in claim 1, wherein the gas oil has a T₉₅>300° C.
 5. A process as claimed in claim 4, wherein the gas oil is selected from the group consisting of light gas oils (LGO), heavy gas oils (HGO), light cycle oils (LCO), coker gas oils (CGO), Visbroken gas oils (VBGO), and mixtures thereof.
 6. A process as claimed in claim 1, wherein a sulphur level of less than 30 ppm is obtained.
 7. A process as claimed in claim 1, wherein the gas oil is contacted with hydrogen in a ratio of 150:1 to 350:1 (nm³/m³, measured at 0° C.).
 8. A process as claimed in claim 1, wherein the gas oil and hydrogen are pre-heated prior to contacting the catalyst.
 9. A process as claimed in claim 1, wherein the catalyst employed in step a) comprises molybdenum and/or tungsten.
 10. A process as claimed in claim 9, wherein the catalyst also comprises cobalt and/or nickel.
 11. A process as claimed in claim 1, wherein a sulphur level of less than 10 ppm is obtained. 